TI Precision Labs - Op Amps: Electrical Overstress (EOS) 3

Hello and welcome to the video for the TI Precision Lab Discussing Electrical Overstress REOS Part 3. In this video we'll show how to select components for EOS protection. We will use the op amp data sheet absolute maximum specifications and application circuit operating conditions to select the appropriate TVS diode and current limiting resistors.
The objective in selecting the TVS diode is to make sure that the diode is off and has minimal leakage during normal operation, but turns on and limits the supply voltage under overstress conditions. The objective in selecting the resistor is to limit the input current to less than 10 milliamps.
Let's review some of the characteristics of TVS diodes, given in EOS 1. The most critical spec for TVS selection is the reverse standoff voltage RVR. Looking at the IV characteristic on the right, we see that VR is the highest operating voltage of the TVS where it still has low leakage current IR, typically around 1 microamp. This is the off stage for the TVS.
If higher voltages are applied, the TVS will reach its reverse breakdown voltage where it will turn on and limit voltage, as well as conduct significant current. Finally, the clamping voltage VC is the voltage across the TVS where the peak pulse current, or IPP, is flowing. These three points on the IV curve are what we'll use to select a TVS.
Both the amplifier operating conditions and the TVS diode specifications need to be considered when selecting the TVS diode. First, let's consider the amplifier specifications. The specified power supply range is the range under which the device can be used while maintaining its specified data sheet performance.
In this example, the OPA 192 specified power supply range is from plus or minus 2.25 volts to plus or minus 18 volts. If a supply voltage of plus or minus 18 volts is used for the amplifier, then the TVS diode needs to remain off and have low leakage.
Remember that the reverse standoff voltage specification on the TVS diode, RVR, is the maximum voltage that can be applied to a TVS diode where the maximum reverse leakage current is valid. This example diode was selected to match the maximum specified supply voltage from the amplifier. VR is specified at 18 volts. And the associated maximum reverse leakage is 5 microamps.
This means that if the reverse voltage applied across the TVS diode is 18 volts or less, the leakage current will never be above 5 microamps. Normally the standoff voltage is selected according to the maximum operating conditions of the circuit. So if the amplifier was operated at a lower voltage, the reverse standoff voltage would need to be selected accordingly.
Please note the difference between the specified voltage range and absolute maximum range of an op amps power supply. The specified voltage range is the range where the specs are valid, and the range under which the device is designed to operate. The absolute maximum range is the range that can be applied to the device before damage is caused. In this example, we are discussing the specified range of plus or minus 18 volts. The absolute maximum for this device is plus or minus 20 volts.
Let's take a look at the absolute maximum specifications for the OPA 192. Recall from the last slide that the OPA 192's operating voltage is plus or minus 18 volts. The absolute maximum for this device is plus or minus 20 volts. So if the amplifier supply is normally at plus or minus 18 volts, then the TVS diode must be fully off at 18 volts. However, the TVS diode needs to turn on to protect the amplifier before the supply reaches the absolute maximum of plus or minus 20 volts.
The fault voltage is the voltage across the TVS diode when it is turned on and protecting the device. The fault voltage is dependent on the current that flows during the over voltage fault condition. Ideally, this fault voltage should be kept lower than the absolute maximum voltage.
Let's look again at the TVS diode IV curve to better understand this concept. In our example, we are targeting 18 volts as the reverse standoff voltage, because this is the normal operating voltage that will be applied to the op amp. VBR is the breakdown voltage, the point at which the device is just beginning to turn on.
Typically, 1 milliamp of current flows at the breakdown voltage. The clamp voltage VC is the voltage across the TVS diode with maximum reverse current flowing through it. Depending on the current expected during the fault condition, the fault voltage will be somewhere between VBR and VC. One way to estimate the fault current is to consider the maximum supply current and add margin.
In this example we will estimate the fault current at 2 amps. Since VC and VBR are specified, it is possible to interpolate between the two points to determine the fault voltage at a specific fault current, or 2 amps, in this example.
Here we show how to use linear interpolation to find the fault voltage across the example TVS diode for 2 amps of fault current. The equation is the standard straight line in the form I equals m times V plus b. Solving for the slope, m, we divide the change in current over the change in voltage for the breakdown and clamp points.
In this example we use the maximum values for these points so that the solution is conservative. After solving for the slope, we rearrange the equation and solve for the y-axis intercept, RB. Finally, we substitute this information back into the equation and solve for the fault voltage at 2 amps. In this case, the fault voltage is 23.14 volts.
We must now compare the tedious voltages to the op amp specifications. Recall that the TVS reverse standoff of 18 volts was selected to match the op amp operating supply voltage of 18 volts. We determined that the fault voltage is actually 23.14 volts using linear interpolation between known points on its IV curve. For best protection, the fault voltage should be less than the op amp absolute maximum voltage. Unfortunately, this TVS fault voltage is above the op amp absolute maximum of 20 volts.
In the second option, the op amp supply voltage is reduced to 15 volts. This gives more margin between the maximum supply and the absolute maximum voltage. A new TVS is selected with the 15 volts reverse standoff, which results in a fault voltage of 19.2 volts, less than the op amp absolute maximum. Thus, this TVS diode will effectively protect against EOS events.
It should be noted that in some cases it is not possible to find a TVS diode that meets both the operating and absolute maximum conditions. In this case, it is recommended to still use the TVS in spite of the fact that it will not limit the fault voltage to an EOS-safe level. The reason is that some protection is always better than no protection.
Ideally, the TVS reverse standoff voltage and fault voltage would be very close to each other, since op amps often have operating conditions which are near the absolute maximum voltage. However, this isn't always the case. Thankfully, we can consider TVS diodes with different power ratings. Increasing the power rating increases the slope of the curve in the reverse breakdown region, which will move the fault voltage closer to the reverse standoff voltage.
Note that the power rating refers to the peak power dissipated during a 1 millisecond pulse. In this example we compare a 1,500 watt TVS diode to a 400 watt TVS diode. You can see from the curve on the right that at 2 amps, the 400-watt device has a fault voltage of 19.2 volts, while the 1,500-watt device has a fault voltage of only 18.6 volts. This reduction can be very helpful when selecting a TVS.
However, there is a disadvantage to high power TVS diodes. They're actually quite large compared to most op amp packages. The diagram on the left compares the size of an op amp in a 5-pin SOT package, a 400-watt TVS in an SMA package, and a 1,500-watt TVS in an SMC package.
Comparing the results for TVS diodes with different power ratings, we clearly see that the fault voltage is significantly lower for the higher power TVS diode in option 2. In this case, both options will protect against EOS, because the fault voltage is lower than the absolute maximum of the op amp. However, the higher power option adds additional margin.
Furthermore, if operating conditions were closer to the absolute maximum conditions, this might be the only option. The last step to developing effective EOS protection is to choose the current limiting resistance in series with the input. First, pick a worst case EOS voltage-- 100 volts in this example. Second, do a voltage walk through the path of current flow to determine the voltage drop across the resistor. In this example, the 100 volts is distributed across RN, D3, and D7.
D3 has about 0.7 volts of voltage drop. And D7 has a 19.2 volts dropped based on its fault voltage. So 84.3 volts remains across the resistor.
The absolute maximum current flow into the amplifier before EOS damage is 10 milliamps. Use Ohm's law to calculate the minimum resistor value based on the voltage of 84.3 volts and the maximum current of 10 milliamps, which results in 8.43 kilohms. Note that increasing the resistance will improve the protection, but may cause other performance trade-offs.
That concludes this video. Thank you for watching. Please try the quiz to check your understanding of this video's content.

Details

Date:
March 23, 2015

This is the third of four videos in the TI Precision Labs – Op Amps curriculum that addresses operational amplifier electrical overstress (EOS). In this training, we’ll show how to select components for EOS protection. We will use the op-amp data sheet absolute maximum specifications and application circuit operating conditions to select the appropriate TVS diode and current-limiting resistors.

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